Three Pairs of Enantiomeric Sesquiterpenoids from Syringa pinnatifolia

Three pairs of enantiomeric sesquiterpenoids, (∓)-syringanoid A (1a and 1b) and (±)-pinnatanoids A (2a and 2b) and B (3a and 3b), that represent an unprecedented 5/4/6 tricyclic backbone and a rare 6/7 bicyclic backbone, respectively, were isolated from the peeled stems of Syringa pinnatifolia. The structures were elucidated by extensive spectroscopic analysis, single-crystal X-ray diffraction, a modified Mosher's method, and quantum chemical calculations. A plausible biotransformation pathway for 1-3 was proposed, and their cardiomyocyte-protective and anti-inflammatory activities were evaluated.